Satellite radio navigation systems such as the Global Positioning System (GPS) are valuable tools for the navigation of moving vehicles. These vehicles may be ground or air vehicles, and may be manned or unmanned. Traditionally, GPS antennas for moving vehicles have been microstrip patch antennas which have a low vertical profile and low wind resistance. These antennas are lightweight, inexpensive, and typically only several inches square, and may protrude one or two centimeters above the surface of the vehicle, at most.
Microstrip patch antennas usually comprise just one square or circular metal antenna element attached to a low-loss dielectric substrate. The substrate is mounted on a larger ground plane, which serves as the return path for current induced on the patch element. The microstrip patch antenna performs optimally when it is sized such that the cavity beneath the patch resonates in its fundamental mode (TM100 or TM010) at the frequency of interest. This occurs when the resonant dimension of the patch is approximately one half-wavelength long within the dielectric substrate. Circularly polarized reception is possible when both the TM100 and TM010 modes are excited with equal strength, but with a 90° phase shift. Circular polarization is important since most navigation satellites transmit circularly polarized radiation, and therefore a circularly polarized receive antenna is preferred for optimal system performance. While microstrip patch antennas inherently possess a narrow bandwidth, bandwidth enhancement techniques are possible to allow reception of wideband signals. Wide bandwidth antennas act to improve the accuracy of the navigation position solution.
Typical patch antennas have broad hemispherical radiation patterns, which enable them to acquire and track all GPS satellites above the horizon. However, this broad beam also receives radio energy from below the horizon. For ground or airborne vehicles, this undesired energy typically originates from ground-based radio frequency interference (RFI) sources. In both cases, it is possible for the interference to jam the receiver and render it unusable, thus preventing the user from navigating with the satellite radio navigation system.
Several techniques have been employed to combat these problems associated with RFI. A common approach uses an adaptive phased array of patch antennas arranged in a plane. The signals from multiple antennas are combined adaptively in a manner to reduce the interference. One way to combine the signals is to form distinct narrow beams directed at each of the navigation satellites. This is called a beam-steered controlled radiation pattern antenna (CRPA). Another way to combine signals from multiple antennas is to place nulls in the directions of the interference sources. This is called a null-steering CRPA. For an N-element array, the null-steering CRPA can steer N−1 nulls simultaneously. The depth of each null is limited by the number of nulls that are active simultaneously. The beam-steered and null-steering CRPAs must adaptively compute, in real time, the appropriate weightings for the signal combining. This requires expensive external hardware and software, and specialized receiver design. Phased arrays of patch antennas are also quite large, requiring multiple patch antennas sufficiently separated from each other.
Adaptive antenna arrays do not integrate directly with existing aviation or consumer navigation equipment, and therefore their use requires significant retrofit of the vehicle's radio navigation system. While adaptive antenna arrays have impressive interference suppression performance, they are not well suited for consumer or commercial use due to their cost, complexity, and large size. They find use largely in military anti-jam applications.
Various antennas have been designed to yield higher bandwidths. For example, U.S. Pat. No. 5,319,378 issued to Nalbandian et al. discloses a multi-band antenna consisting of a patch element supported by several dielectric layers. This multi-band antenna can be excited in higher order modes allowing multi-frequency operation. However, it does not suppress interference.
U.S. Pat. No. 5,003,318 issued to Berneking et al. describes an antenna comprising a multi-layer patch antenna system designed primarily to increase the receive bandwidth and provide dual-frequency operation. Two circular patch elements are resonant on closely separated frequencies and are excited simultaneously, which broadens the antenna's total frequency response. An adaptive nulling processor is required for suppressing interference. In addition, the vertical structure of this antenna is very complex, requiring specialized manufacturing techniques.
U.S. Pat. No. 5,712,641 issued to Casabona et al. discloses an interference cancellation system for GPS receivers that makes use of polarization diversity. The two orthogonal polarization components are treated independently in two separate channels, and are adaptively weighted and combined in a manner to suppress undesired interference. This antenna system requires external hardware and software to adaptively compute the weightings of the two polarization signals.
U.S. Pat. No. 5,461,387 issued to Weaver describes a direction finding antenna consisting of a four-arm spiral that may be excited in two different modes. Mode 1 maintains a 90° phase shift between the arms and is a very broad pattern useful for receiving all navigation satellites in view. Mode 2 creates a 180° phase shift between the arms, resulting in a null in the direction of the antenna axis. Again, this antenna requires external hardware and software to compute the relative amplitude and phase between the two modes. No mention is made of any interference suppression capabilities of the antenna. Moreover, because of its design, this antenna is not practical for external use on high-speed vehicles.
U.S. Pat. No. 6,252,553 issued to Solomon discloses a multi-mode patch antenna that can steer a null in the direction of a jammer. It employs a single microstrip patch operating in both the fundamental mode (TM100 and TM100 phased 90° apart) and the second order mode (TM200 and TM020). The amplitude and phase of these two modes may be adaptively combined in a manner to suppress interference arriving from one direction. One drawback of this design is that only one jammer may be suppressed. In addition, and more importantly, complex external hardware and software are needed to adaptively compute the proper amplitude and phase for the signal combiner. This antenna is a simpler version of the null-steering CRPA, but still requires complex external hardware. As such, it cannot be integrated directly with existing vehicular radio navigation systems.
An antenna capable of switching between two radiation patterns is disclosed in Ngamjanyapom et al., “Switched-beam single patch antenna,” Electronics Letters, Vol. 38 (2002), No. 1. The antenna comprises a single conducting patch above a ground plane. A combination of forward and reverse biased PIN diodes connect the patch to the ground plane. By reversing the voltage bias to the antenna, the radiation pattern of the patch is switched between two different azimuth radiation patterns: a north-south beam and an east-west beam. Both beams have the same elevation radiation pattern, which does not significantly suppress radiation at or below the horizon. Moreover, both beams significantly suppress signals near zenith, blocking desired signals from GPS satellites overhead. Thus, this antenna would not be useful for improving the signal to interference ratio in satellite radio navigation systems.
U.S. Pat. No. 5,486,836 to Kuffner et al. teaches a dual patch antenna system where the two antennas are physically separated, e.g., located in different parts of the transceiver. This spatial separation of the antennas provides spatial diversity and also isolates the antennas to avoid mutual coupling. One of the two antennas can be selected with an RF switch implemented with PIN diodes. The antenna system, however, has no properties that address the specific problems associated with interference from ground-based RF signal sources in GPS navigational systems. Moreover, because the two antennas must be separated to avoid mutual coupling, the antenna system taught by Kuffner et al. is not compact or integrated.
U.S. Pat. No. 5,877,726 to Kudoh et al. teaches an array antenna system wherein array elements are spatially separated from each other in a diagonal configuration in a common plane. The diagonal staggered arrangement of the elements increases the physical separation between elements and reduces the mutual coupling (i.e., interference) between antenna elements. The antenna system does not have any specific properties that help solve any of the problems associated with interference from ground-based RF signal sources in GPS navigational systems. In addition, the antenna array is not compact since the antennas must be physically separated.
There is therefore a need for a simple, inexpensive, low-profile patch antenna that is able to mitigate radio frequency and multi-path interference to satellite radio navigation systems without requiring modifications to currently installed systems.